Modern cargo aircraft, in particular of the type Boeing 747, have extraordinarily large cargo holds into which containers are loaded. The loading situations can vary widely. The items being stored in the hold are not only containers tailored exactly to the dimensions of the hold, which can be locked into position at prespecified places by means of elements provided there. Instead, it is also necessary to load items, the sizes of which require that they be conveyed to special sites for storage. Very large items of freight can even need to be brought in through a side door and turned around within the cargo hold.
In all cases, however, the loading must proceed as rapidly as possible. The time taken for loading increases the costs to the operator of the cargo aircraft. When the conditions for loading are not as they should be, or when extreme environmental situations such as especially high or low temperatures, dust/sand, moisture etc. interfere with the function of operating systems needed for loading, in particular PDUs, the loading process can be delayed. In such cases it should always be kept in mind that not every airport has available a staff of well-trained operators with extensive experience. Hence even simple errors in operation, which can cause failure of entire systems, are not uncommon.